The present invention relates to an electroless copper plating method of reducing the concentration of byproduct ions and deposition of byproducts in a plating solution, a device thereof, and an application thereof.
The usual electroless copper plating solution contains copper ions, a copper ion complex agent, a copper ion reducing agent, and a pH conditioner, which must be supplied as the plating advances. However, this supply increases anions in pairs with copper ions, oxidant ions of the copper ion reducing agent, and cations of the pH conditioner in the plating solution.
These ions increasing in the plating solution will deteriorate the physical properties of the plated layer, particularly the elongation ratio of the plated layer, reducing the reliability of the plated layer. Further, it reduces the stability of the plating solution and causes abnormal deposition and autolysis.
Conventionally, to avoid such problems and to maintain a constant salt concentration in the plating solution, various plating techniques have been used, such as changing plating solutions at short-time intervals, and continuously adding a new plating liquid to the plating solution in use. However, these approaches require a large quantity of expensive copper plating solution and a lot of labor and money to dispose of liquid wastes.
Japanese Non-examined Patent Publication No. 56-136967 (1981) has disclosed a method of continuously removing such cumulative ions by an electro dialysis. This method requires complicated operations to maintain the optimum pH for dialysis. Further, the ion selecting membrane which allows plating blocking ions only to penetrate the membrane is mechanically weak, difficult to maintain, and too expensive.
Japanese Non-examined Patent Publication No. 7-268638 (1995) has disclosed a plating method characterized by selecting the metallic ion reducing agent and the pH conditioner in the non-electro plating solution so that the oxidant ion of the metallic ion reducing agent and the cation of the pH conditioner may react into an insoluble salt to prevent oxidant ions of said metallic ion reducing agent from increasing in said plating solution.
However, this method is not effective to prevent an increase of anions in pairs with metallic ions. The increase of the anions will deteriorate the plating characteristics. This method also suggests that the use of copper oxide or copper hydroxide in copper plating will suppress characteristic deterioration of the plating solution. In this case, however, the solubility of the copper oxide or copper hydroxide has a great influence.
Solid copper oxide or copper hydroxide is usually added to the plating solution. If the solid is not dissolved completely into the plating solution, the particles left undissolved are plated as the cores. This causes abnormal deposition or autolysis. Further, copper oxide and copper hydroxide are more expensive than copper sulfate as copper ion sources, which is conventionally used for copper plating. That's the reason why copper oxide and copper hydroxide have not been put in practical use.
Japanese Non-examined Patent Publication No. 7-286279 (1995) has disclosed a method of adding barium hydroxide to the non-electro plating solution and removing excessive sulfuric ions as barium sulfate from the plating solution. However, this method using formalin (35% aqueous formaldehyde solution) cannot avoid a consequent increase of oxidant ions of the copper ion reducing agent in the copper plating solution. The oxidant ions of the copper ion reducing agent in this method are formic ions and cannot be removed because barium formate has too great a solubility to be precipitated.
Further, this method does not blow air into the solution while adding barium hydroxide into the plating solution. When alkaline barium hydroxide is added to the plating solution, the pH value of the plating solution becomes higher. In electroless copper plating, the plating solution becomes unstable when its pH goes too high. Consequently, copper may deposit on unwanted places. This abnormal deposition on printed circuits and the like may cause short-circuits, reducing the yield of the products. Deposition on the walls of the plating bath may drastically deteriorate the workability.
The conventional plating equipment is usually designed to directly add copper ions, the copper-ion reducing agent, and the pH conditioner into the plating bath. However, this equipment cannot be free from the floating of solid particles of insoluble salt in the plating solution. The floating solid particles when deposited on wiring boards may cause abnormal deposition on the boards.
If such a solid particle is caught in a through-hole on a printed wiring board, it prevents part of the through-hole from being plated, causing a discontinuity of the wiring (which is termed “through-hole void”).
In continuous electroless copper plating, byproduct ions such as anions in pairs with copper ions and oxidant ions of the copper ion reducing agent increase in the plating solution. This increasing of the byproduct ions prevent the electroless copper plating reaction from forming normal plating layers and reduces the quality of the plated layer. This not only reduces the mechanical properties of the plated layer, but also causes abnormal deposition of metal on unwanted locations. Up to now, there have been disclosed no effective electroless copper plating method of preventing the increase of plating blocking ions or removing the increased plating blocking ions and refreshing the plating solution.